Newspapers in Cameroon are subject to considerable official restrictions
Cameroonian editor Germain Ngota has died in prison in the capital, Yaounde.
He was the managing editor of the Cameroon Express and one of three reporters detained in March on charges of fraud and using false documents.
An adviser to the Cameroonian journalists' union (SNJC) said Mr Ngota was not given any medical treatment during his detention.
Mr Ngota was reportedly suffering from high blood pressure. The authorities have not commented on his death.
"I am shocked and appalled by his death," SNJC's Henriette Ekwee said.
The New York-based Committee to Protect Journalists (CPJ) also said it held the authorities responsible for Mr Ngota's death.
"The authorities failed to provide Ngota with adequate medical treatment despite repeated requests by his family and colleagues," the CPJ's Tom Rhodes said in a statement.
According to the CPJ, Mr Ngota had been investigating corruption allegations involving a presidential adviser and the state-run oil company at the time of his arrest.
Newspapers in Cameroon are subject to considerable official restrictions and tough libel legislation is in place.
In 2006 the media rights body Reporters Without Borders noted that: "Draconian laws regularly put journalists behind bars."